OSCA Genes in Bread Wheat: Molecular Characterization, Expression Profiling, and Interaction Analyses Indicated Their Diverse Roles during Development and Stress Response

Fighting for Survival at the Stomatal Gate

Stomata serve as the battleground between plants and plant pathogens. Plants can perceive pathogens, inducing closure of the stomatal pore, while pathogens can overcome this immune response with their phytotoxins and elicitors. In this review, we summarize new discoveries in stomata-pathogen interactions. Recent studies have shown that stomatal movement continues to occur in a close-open-close-open pattern during bacterium infection, bringing a new understanding of stomatal immunity. Furthermore, the canonical pattern-triggered immunity pathway and ion channel activities seem to be common to plant-pathogen interactions outside of the well-studied Arabidopsis-Pseudomonas pathosystem. These developments can be useful to aid in the goal of crop improvement. New technologies to study intact leaves and advances in available omics data sets provide new methods for understanding the fight at the stomatal gate. Future studies should aim to further investigate the defense-growth trade-off in relation to stomatal immunity, as little is known at this time.

Cloning a novel reduced-height (Rht) gene TaOSCA1.4 from a QTL in wheat

Reducing plant height (PH) is one of the core contents of the "Green Revolution", which began in the 1960s in wheat. A number of 27 reduced-height (Rht) genes have been identified and a great number of quantitative trait loci (QTLs) for PH have been mapped on all 21 chromosomes. Nonetheless, only several genes regulated PH have been cloned. In this study, we found the interval of QTL QPh-1B included an EST-SSR marker swes1079. According to the sequence of swes1079, we cloned the TaOSCA1.4 gene. We developed a CAPS marker to analyze the variation across a natural population. The result showed that the PH was significantly different between the two haplotypes of TaOSCA1.4-1B under most of the 12 environments and the average values of irrigation and rainfed conditions. This result further demonstrated that TaOSCA1.4 was associated with PH. Then, we validated the TaOSCA1.4 via RNAi technology. The average PHs of the wild-type (WT), RNAi lines 1 (Ri-1) and 2 (Ri-2) were 94.6, 83.6 and 79.2 cm, respectively, with significant differences between the WT and Ri-1 and Ri-2. This result indicated that the TaOSCA1.4 gene controls PH. TaOSCA1.4 is a constitutively expressed gene and its protein localizes to the cell membrane. TaOSCA1.4 gene is a member of the OSCA gene family, which regulates intracellular Ca2+ concentration. We hypothesized that knock down mutants of TaOSCA1.4 gene reduced regulatory ability of Ca2+, thus reducing the PH. Furthermore, the cell lengths of the knock down mutants are not significantly different than that of WT. We speculate that TaOSCA1.4 gene is not directly associated with gibberellin (GA), which should be a novel mechanism for a wheat Rht gene.

Investigation of two-pore K+ (TPK) channels in Triticum aestivum L. suggests their role in stress response

Two-pore K+ (TPK) channels are voltage-independent and involved in stress response in plants. Herein, we identified 12 TaTPK genes located on nine chromosomes in the Triticum aestivum genome. The majority of TaTPK genes comprised two exons. Each TaTPK channel comprised four transmembrane (TM) helices, N- and C-terminal ion-channel domains, two EF-hand domains and one 14-3-3 binding site. Additionally, highly conserved 'GYGD' motif responsible for K+ ion specificity, was found in between the TMs in both the ion-channel domains. Nine TaTPK channels were predicted to be localised at the plasma membrane, while three were vacuolar. The protein-protein and protein-chemical interactions indicated the coordinated functioning of the TaTPK channels with the other K+ transporters and their possible interaction with the Ca2+-signaling pathway. Expression studies suggested their importance in both vegetative and reproductive tissues development. Significantly modulated expression of various TaTPK genes during heat, drought, combined heat and drought and salt stresses, and after fungal infestation, depicted their function in stress responses. The miRNAs and transcription factors interaction analyses suggested their role in the hormone, light, growth and development-related, and stress-responsive signaling cascades. The current study suggested vital functions of various TaTPK genes, especially in stress response, and would provide an opportunity for their detailed characterization in future studies.

Comprehensive Analysis of WUSCEL-Related Homeobox Gene Family in Ramie (Boehmeria nivea) Indicates Its Potential Role in Adventitious Root Development

A WUSCHEL-related homeobox (WOX) gene family has been implicated in promoting vegetative organs to embryonic transition and maintaining plant embryonic stem cell identity. Using genome-wide analysis, we identified 17 candidates, WOX genes in ramie (Boehmeria nivea). The genes (BnWOX) showed highly conserved homeodomain regions typical of WOX. Based on phylogenetic analysis, they were classified into three distinct groups: modern, intermediate, and ancient clades. The genes displayed 65% and 35% collinearities with their Arabidopsis thaliana and Oryza sativa ortholog, respectively, and exhibited similar motifs, suggesting similar functions. Furthermore, four segmental duplications (BnWOX10/14, BnWOX13A/13B, BnWOX9A/9B, and BnWOX6A/Maker00021031) and a tandem-duplicated pair (BnWOX5/7) among the putative ramie WOX genes were obtained, suggesting that whole-genome duplication (WGD) played a role in WOX gene expansion. Expression profiling analysis of the genes in the bud, leaf, stem, and root of the stem cuttings revealed higher expression levels of BnWOX10 and BnWOX14 in the stem and root and lower in the leaf consistent with the qRT-PCR analysis, suggesting their direct roles in ramie root formation. Analysis of the rooting characteristics and expression in the stem cuttings of sixty-seven different ramie genetic resources showed a possible involvement of BnWOX14 in the adventitious rooting of ramie. Thus, this study provides valuable information on ramie WOX genes and lays the foundation for further research.

Genome-Wide Identification and Analysis of the ABCF Gene Family in Triticum aestivum

The ATP-binding cassette (ABC) superfamily of proteins is a group of evolutionarily conserved proteins. The ABCF subfamily is involved in ribosomal synthesis, antibiotic resistance, and transcriptional regulation. However, few studies have investigated the role of ABCF in wheat (Triticum aestivum) immunity. Here, we identified 18 TaABCFs and classified them into four categories based on their domain characteristics. Functional similarity between Arabidopsis and wheat ABCF genes was predicted using phylogenetic analysis. A comprehensive genome-wide analysis of gene structure, protein motifs, chromosomal location, and cis-acting elements was also performed. Tissue-specific analysis and expression profiling under temperature, hormonal, and viral stresses were performed using real-time quantitative reverse transcription polymerase chain reaction after randomly selecting one gene from each group. The results revealed that all TaABCF genes had the highest expression at 25 °C and responded to methyl jasmonate induction. Notably, TaABCF2 was highly expressed in all tissues except the roots, and silencing it significantly increased the accumulation of Chinese wheat mosaic virus or wheat yellow mosaic virus in wheat leaves. These results indicated that TaABCF may function in response to viral infection, laying the foundation for further studies on the mechanisms of this protein family in plant defence.

Genome-wide investigation and expression analysis of OSCA gene family in response to abiotic stress in alfalfa

Alfalfa is an excellent leguminous forage crop that is widely cultivated worldwide, but its yield and quality are often affected by drought and soil salinization. Hyperosmolality-gated calcium-permeable channel (OSCA) proteins are hyperosmotic calcium ion (Ca2+) receptors that play an essential role in regulating plant growth, development, and abiotic stress responses. However, no systematic analysis of the OSCA gene family has been conducted in alfalfa. In this study, a total of 14 OSCA genes were identified from the alfalfa genome and classified into three groups based on their sequence composition and phylogenetic relationships. Gene structure, conserved motifs and functional domain prediction showed that all MsOSCA genes had the same functional domain DUF221. Cis-acting element analysis showed that MsOSCA genes had many cis-regulatory elements in response to abiotic or biotic stresses and hormones. Tissue expression pattern analysis demonstrated that the MsOSCA genes had tissue-specific expression; for example, MsOSCA12 was only expressed in roots and leaves but not in stem and petiole tissues. Furthermore, RT-qPCR results indicated that the expression of MsOSCA genes was induced by abiotic stress (drought and salt) and hormones (JA, SA, and ABA). In particular, the expression levels of MsOSCA3, MsOSCA5, MsOSCA12 and MsOSCA13 were significantly increased under drought and salt stress, and MsOSCA7, MsOSCA10, MsOSCA12 and MsOSCA13 genes exhibited significant upregulation under plant hormone treatments, indicating that these genes play a positive role in drought, salt and hormone responses. Subcellular localization results showed that the MsOSCA3 protein was localized on the plasma membrane. This study provides a basis for understanding the biological information and further functional analysis of the MsOSCA gene family and provides candidate genes for stress resistance breeding in alfalfa.

Genome-Wide Identification and Functional Analysis of the TIFY Family Genes in Response to Abiotic Stresses and Hormone Treatments in Tartary Buckwheat (Fagopyrum tataricum)

TIFY is a plant-specific gene family with four subfamilies: ZML, TIFY, PPD, and JAZ. Recently, this family was found to have regulatory functions in hormone stimulation, environmental response, and development. However, little is known about the roles of the TIFY family in Tartary buckwheat (Fagopyrum tataricum), a significant crop for both food and medicine. In this study, 18 TIFY family genes (FtTIFYs) in Tartary buckwheat were identified. The characteristics, motif compositions, and evolutionary relationships of the TIFY proteins, as well as the gene structures, cis-acting elements, and synteny of the TIFY genes, are discussed in detail. Moreover, we found that most FtTIFYs responded to various abiotic stresses (cold, heat, salt, or drought) and hormone treatments (ABA, MeJA, or SA). Through yeast two-hybrid assays, we revealed that two FtTIFYs, FtTIFY1 and FtJAZ7, interacted with FtABI5, a homolog protein of AtABI5 involved in ABA-mediated germination and stress responses, implying crosstalk between ABA and JA signaling in Tartary buckwheat. Furthermore, the overexpression of FtJAZ10 and FtJAZ12 enhanced the heat stress tolerance of tobacco. Consequently, our study suggests that the FtTIFY family plays important roles in responses to abiotic stress and provides two candidate genes (FtJAZ10 and FtJAZ12) for the cultivation of stress-resistant crops.

Duplicate Genes Contribute to Variability in Abiotic Stress Resistance in Allopolyploid Wheat

Gene duplication is a universal biological phenomenon that drives genomic variation and diversity, plays a crucial role in plant evolution, and contributes to innovations in genetic engineering and crop development. Duplicated genes participate in the emergence of novel functionality, such as adaptability to new or more severe abiotic stress resistance. Future crop research will benefit from advanced, mechanistic understanding of the effects of gene duplication, especially in the development and deployment of high-performance, stress-resistant, elite wheat lines. In this review, we summarize the current knowledge of gene duplication in wheat, including the principle of gene duplication and its effects on gene function, the diversity of duplicated genes, and how they have functionally diverged. Then, we discuss how duplicated genes contribute to abiotic stress response and the mechanisms of duplication. Finally, we have a future prospects section that discusses the direction of future efforts in the short term regarding the elucidation of replication and retention mechanisms of repetitive genes related to abiotic stress response in wheat, excellent gene function research, and practical applications.

Molecular Mechanisms of Plant Defense against Abiotic Stress

The climatic changes and anthropogenic factors in recent decades (global warming, drought, salinity, extreme temperature, environmental pollution) have led to an increase in the negative impact of environmental factors on plants. Abiotic stress strongly influences the important processes of plants and thus affects their growth and development. The effects of stressors on the plants depend on the intensity, frequency, and duration of stress, plant species as well as a combination of various stressors. Plants have developed different mechanisms to limit adverse environmental conditions. In the publications in this Special Issue, Molecular Mechanisms of Plant Defense against Abiotic Stress, new information on plant defense mechanisms against abiotic and biotic stress is presented. The studies help us better understand plants' protection mechanisms again global climate change.

Genome-Wide Identification and Expression Analysis of the SWEET Gene Family in Annual Alfalfa (Medicago polymorpha)

SWEET (Sugars will eventually be exported transporter) proteins are a group of sugar transporters that are involved in sugar efflux, phloem loading, reproductive development, plant senescence, and stress responses. In this study, 23 SWEET transporter members were identified in the Medicago polymorpha genome, heterogeneously distributed on seven chromosomes. These MpSWEET genes were divided into four subfamilies, which showed similar gene structure and motif composition within the same subfamily. Seventeen MpSWEET genes encode seven transmembrane helices (TMHs), and all MpSWEET proteins possess conserved membrane domains and putative serine phosphorylation sites. Four and three pairs of MpSWEET genes were predicted to be segmentally and tandemly duplicated, respectively, which may have contributed to their evolution of M. polymorpha. The results of microarray and RNA-Seq data showed that some MpSWEET genes were specifically expressed in disparate developmental stages (including seedling stage, early flowering stage, and late flowering stage) or tissues such as flower and large pod. Based on protein network interaction and expression patterns of MpSWEET genes, six MpSWEET genes were selected for further quantitative real-time PCR validation in different stress treatments. qRT-PCR results showed that MpSWEET05, MpSWEET07, MpSWEET12, MpSWEET15, and MpSWEET21 were significantly upregulated for at least two of the three abiotic stress treatments. These findings provide new insights into the complex transcriptional regulation of MpSWEET genes, which facilitates future research to elucidate the function of MpSWEET genes in M. polymorpha and other legume crops.

Genome-Wide Identification and Expression Analysis of Calmodulin (CaM) and Calmodulin-Like (CML) Genes in the Brown Algae Saccharina japonica

Calmodulins (CaMs) and Calmodulin-like proteins (CMLs) are vital in plant growth, development, and stress responses. However, CaMs and CMLs have not been fully identified and characterized in brown algae, which has been evolving independently of the well-studied green plant lineage. In this study, whole-genome searches revealed one SjCaM and eight SjCMLs in Saccharina japonica, and one EsCaM and eleven EsCMLs in Ectocarpus sp. SjCaM and EsCaM encoded identical protein products and shared 88.59-89.93% amino acid identities with Arabidopsis thaliana AtCaMs, thereby indicating that brown algae CaMs retained a similar Ca2+ sensors function as in plants. The phylogenetic and gene structure analysis results showed that there was significant divergence in the gene sequences among brown algae CMLs. Furthermore, evolutionary analysis indicated that the function of brown alga CMLs was relatively conserved, which may be related to the fact that brown algae do not need to face complex environments like terrestrial plants. Regulatory elements prediction and the expression analysis revealed the probable functioning of SjCaM/CML genes in gametophyte development and the stress response in S. japonica. In addition, the SjCaM/SjCMLs interacting proteins and chemicals were preliminarily predicted, suggesting that SjCaM/SjCMLs might play putative roles in Ca2+/CaM-mediated growth and development processes and stimulus responses. Therefore, these results will facilitate our understanding of the evolution of brown algae CaMs/CMLs and the functional identification of SjCaM/SjCMLs.

Comprehensive Genomic Analysis of SnRK in Rosaceae and Expression Analysis of RoSnRK2 in Response to Abiotic Stress in Rubus occidentalis

The sucrose nonfermenting 1-related protein kinase (SnRK) plays an important role in responding to abiotic stresses by phosphorylating the target protein to regulate various signaling pathways. However, little is known about the characteristics, evolutionary history, and expression patterns of the SnRK family in black raspberry (Rubus occidentalis L.) or other Rosaceae family species. In this study, a total of 209 SnRK genes were identified in 7 Rosaceae species and divided into 3 subfamilies (SnRK1, SnRK2, and SnRK3) based on phylogenetic analysis and specific motifs. Whole-genome duplication (WGD) and dispersed duplication (DSD) were considered to be major contributions to the SnRK family expansion. Purifying selection was the primary driving force in the SnRK family evolution. The spatial expression indicated that the RoSnRK genes may play important roles in different tissues. In addition, the expression models of 5 RoSnRK2 genes in response to abiotic stresses were detected by qRT-PCR. The proteins encoded by RoSnRK2 genes localize to the cytoplasm and nucleus in order to perform their respective functions. Taken together, this study provided an analysis of the SnRK gene family expansion and evolution, and contributed to the current knowledge of the function of 5 RoSnRK2 genes, which in turn expanded understanding of the molecular mechanisms of black raspberry responses to abiotic stress.

Exploration of Piezo Channels in Bread Wheat (Triticum aestivum L.)

Piezo channels belong to an important class of cell membrane-bound, Ca2+-permeable, mechanosensitive channels consisting of a pore and multiple transmembrane helices. In plants, the functional aspects of Piezo channels have been less studied than other mechanosensitive channels. However, a few studies that have been carried out indicate the involvement of Piezo channels in stress response and developmental processes. In our analysis, we identified a total of three Piezo genes in the Triticum aestivum genome. The phylogenetic analysis revealed the monocot and dicot-specific clustering of Piezo proteins. The gene and protein structure analysis indicated their conserved architecture. The promoter region of each of the three Piezo genes contained light-, growth-and development-, hormone-, and stress-responsive cis-regulatory elements. Moreover, the differential expression of Piezo genes in tissue developmental stages and under abiotic and biotic stress conditions indicated their probable role in plant growth and development and various stresses. The quantitative real-time polymerase chain reaction (qRT-PCR) analysis suggested that TaPiezo1-D might be involved in Ca2+ homeostasis. In addition, protein–protein interaction indicated their precise role in glucose, hormone and stress responses. The miRNA interaction analysis further suggested their participation in signaling cascades and biological processes. The present study will extend our understanding about Piezo channels in Ca2+ mediated signaling in plants under various stresses and provide a path for the functional validation of TaPiezo genes in future research.

Genome-Wide Identification of the Rose SWEET Gene Family and Their Different Expression Profiles in Cold Response between Two Rose Species

Sugars Will Eventually be Exported Transporter (SWEET) gene family plays indispensable roles in plant physiological activities, development processes, and responses to biotic and abiotic stresses, but no information is known for roses. In this study, a total of 25 RcSWEET genes were identified in Rosa chinensis 'Old Blush' by genome-wide analysis and clustered into four subgroups based on their phylogenetic relationships. The genomic features, including gene structures, conserved motifs, and gene duplication among the chromosomes of RcSWEET genes, were characterized. Seventeen types of cis-acting elements among the RcSWEET genes were predicted to exhibit their potential regulatory roles during biotic and abiotic stress and hormone responses. Tissue-specific and cold-response expression profiles based on transcriptome data showed that SWEETs play widely varying roles in development and stress tolerance in two rose species. Moreover, the different expression patterns of cold-response SWEET genes were verified by qRT-PCR between the moderately cold-resistant species R. chinensis 'Old Blush' and the extremely cold-resistant species R. beggeriana. Especially, SWEET2a and SWEET10c exhibited species differences after cold treatment and were sharply upregulated in the leaves of R. beggeriana but not R. chinensis 'Old Blush', indicating that these two genes may be the crucial candidates that participate in cold tolerance in R. beggeriana. Our results provide the foundation for function analysis of the SWEET gene family in roses, and will contribute to the breeding of cold-tolerant varieties of roses.

Calcium Channels, OST1 and Stomatal Defence: Current Status and Beyond

Stomatal immunity is regulated by pathogen-associated molecular patterns (PAMPs)- and abscisic acid (ABA)-triggered signalling in different ways. Cytoplasmic Ca²⁺ signature in the guard cells plays a vital function in stomatal immunity, but the mechanism of Ca²⁺ import is unknown. It has been very recently established that the hyperosmolality-gated calcium-permeable channels (OSCAs) and cyclic nucleotide-gated channels (CNGCs) are responsible for the influx of Ca²⁺ in the cytoplasm, which are activated after BIK1-mediated phosphorylation and ABA interaction during PAMPs- and ABA-triggered stomatal immunity in plants, respectively. Further, ABA-triggered OPEN STOMATA1 (OST1) causes the disassembly of microtubules in the guard cells besides activation of S-type anion channels (SLAC1) for the efflux of cytoplasmic anions that leads to stomata closure.